The present invention relates to a method for the preparation of a coprecipitated carbonate of lanthanum, cerium and terbium. More particularly, the invention relates to a method for the preparation of a powder of a coprecipitated carbonate of lanthanum, cerium and terbium consisting of relatively uniform particles of a spheroidal particle configuration and suitable as a starting material of a green-emitting phosphor used in three-wavelength type fluorescent lamps.
Rare earth-based green-emitting phosphors used in fluorescent lamps are prepared by converting a composite oxide of lanthanum, cerium and terbium into a composite phosphate by the dryprocess high-temperature reaction with a reactive phosphate compound followed by a flux treatment of the composite rare earth-based phosphate. It is a requirement in the phosphor-preparing process in general that the powder of the phosphor has a particle size distribution as narrow as possible. In this regard, it is also desirable that the rare earth-based composite oxide of lanthanum, cerium and terbium as the starting material of the green-emitting phosphor has a narrow particle size distribution. Nevertheless, the particle size distribution of the composite oxide products of lanthanum, cerium and terbium produced by the manufacturers of rare earth-based products is not so narrow as desired by the phosphor manufacturers. As to the particle configuration, in addition, the products of the composite oxide of lanthanum, cerium and terbium as produced and supplied by the manufacturers of rare earth-based products mostly have a rod-like particle configuration and are far from spherical particles considered as ideal by the manufacturers of phosphors so that the phosphors produced therefrom also have a rod-like particle configuration.
In order that a composite oxide of lanthanum, cerium and terbium has a narrow particle size distribution and a particle configuration as close to spherical as possible, the precursor of the composite oxide powder, which is a composite carbonate powder of lanthanum, cerium and terbium to be converted into the composite oxide by calcination, also should have a particle size distribution as narrow as possible and a particle configuration close to spherical. As is known, a rare earth-based composite carbonate of lanthanum, cerium and terbium is prepared by the method of coprecipitation in which an aqueous solution of inorganic acid salts, e.g., nitrates, of these rare earth elements in combination is admixed with an aqueous solution of a water-soluble carbonate having alkalinity such as ammonium carbonate, ammonium hydrogen carbonate and the like to coprecipitate the rare earth elements jointly in the form of precipitates of the rare earth carbonates followed by the recovery of the precipitates by filtration, washing with water and drying to give a coprecipitated carbonate of lanthanum, cerium and terbium. Notwithstanding the requirements by the phosphor manufacturers for the particle size distribution and particle configuration of the coprecipitated carbonate of lanthanum, cerium and terbium as mentioned above, no method is known in the prior art by which these requirements can be satisfied.